1. Field of the Invention
The present invention relates to an optical switch for use in various optical systems, and more particularly to an optical switch having the configuration in consideration of the expandability of an optical switch unit or a wavelength selecting unit.
2. Description of the Related Art
As an information transfer speed becomes faster and the amount of information transferred becomes larger, the demands for a wider bandwidth and a larger capacity of a network and a transmission system have been increasing. As their implementation means, an optical network is desired to be built. The core technique for building the optical network is an optical transmission system.
FIGS. 1A and 1B are schematic diagrams showing the configurations of a typical optical network.
FIG. 1A shows an optical crossconnect (optical XC: optical switch) system for use in a single-wavelength optical transmission system. It is intended to accommodate a plurality of input/output optical transmission lines, and to route an optical signal input from an input optical transmission line to a desired output optical transmission line. Each optical signal input from each of input optical transmission lines 1300 is an optical signal having a single wavelength. The example shown in FIG. 1A assumes that the wavelength is .lambda.0. In this configuration, all the wavelengths of the optical signals input from the respective input transmission lines 1300 may be .lambda.0, or may differ depending on the respective transmission lines. However, the wavelengths of optical signals passing through one transmission line are identical because this is a single-wavelength optical transmission system.
Each optical signal input from each of the input optical transmission lines is input to an optical signal processing unit 1302, routed, and output to an output optical transmission line 1301. The optical signal processing unit 1302 is intended to switch an optical signal input from each of the input optical transmission lines 1300, and to output the switched signal to a desired output optical transmission line 1301. That is, the optical signal processing unit 1302 serves as a switch. Its switching capability is controlled by a controlling unit 1303. The controlling unit 1303 obtains the information about routing from an operating system of a network, determines which output optical transmission line 1301 an optical signal input from any of the input optical transmission lines 1300 is to be output to, and implements a desired switching capability by providing a control signal to the optical signal processing unit 1302.
FIG. 1B shows a wavelength-multiplexed optical XC system for use in a wavelength-multiplexing optical transmission system. This system is intended to accommodate a plurality of input/output optical transmission lines, and to route a wavelength-multiplexed optical signal input from an input optical transmission line to a desired output optical transmission line for each wavelength.
That is, this system realizes the switching capability where an optical signal processing unit 1307 routes an optical signal input from an input optical transmission line 1305 and outputs the signal to an output signal optical transmission line 1306 according to the instruction issued from a controlling unit 1308, similar to FIG. 1A. The differences from FIG. 1A are that an optical signal passing through one transmission line is composed of a plurality of wavelengths corresponding to a plurality of channels, and that the optical signal processing unit 1307 must route each optical signal having each of the plurality of wavelengths.
FIGS. 2A, 2B, 3A, and 3B show the typical configurations of the optical signal processing unit used in the wavelength-multiplexed optical XC system.
FIGS. 2A and 2B show the typical configurations of the optical signal processing unit in the wavelength-multiplexed optical XC system adopting an optical switch.
FIG. 2A shows the optical signal processing unit of a fixed wavelength type.
The optical signal input from an input line 1400 is defined to have multiplexed wavelengths .lambda.1 through .lambda.n. The wavelength-multiplexed optical signal is input to a demultiplexer 1401 where the optical signal is demultiplexed into optical signals having respective wavelengths. The demultiplexed optical signals are input to an optical switch unit 1402, routed, and input to a multiplexer 1403. The multiplexer 1403 multiplexes the optical signals, and outputs the multiplexed signals to an output line as a wavelength-multiplexed optical signal. Since the configuration shown in FIG. 2A assumes a fixed wavelength type, the wavelength of the optical signal input from the input line 1400 is output without being converted, multiplexed by a multiplexer 1403, and output to an output line 1404. That is, the wavelength .lambda.1 of an optical signal remains unchanged even when it is output to the output line 1404. As shown in FIG. 2A, a plurality of multiplexers 1403 are arranged in correspondence with a plurality of output lines 1404. However, two or more optical signals having identical wavelengths are not input to one of the plurality of multiplexers 1403. This is because wavelength-multiplexing is required for the optical signals. Accordingly, the optical switch unit 1402 performs routing so that, for example, only one optical signal having a wavelength .lambda.1 is input to one of the plurality of multiplexers 1403.
FIG. 2B shows a typical configuration of the optical signal processing unit of a wavelength converting type. The same constituent elements as those shown in FIG. 2A are denoted by the same reference numerals.
The wavelength-multiplexed optical signal input from the input line 1400 is demultiplexed into optical signals having respective wavelengths by the demultiplexer 1401, and the demultiplexed signals are input to the optical switch unit 1402. The optical switch unit 1402 routes and outputs the optical signals having the respective wavelengths. Since the configuration shown in FIG. 2B is of the wavelength converting type, the optical switch unit 1402 performs routing regardless of the wavelength of an input optical signal. Therefore, if any optical signal output from the output port of the optical switch unit 1402 is input to the multiplexer 1403 where the optical signals are coupled, the coupled signal may contain two or more light beams having identical wavelengths. As a result, proper wavelength multiplexing may sometimes not be performed. Accordingly, a wavelength converting unit 1405 is arranged between the optical switch unit 1402 and the multiplexer 1403, so that the wavelengths of the optical signals to be input to one of the plurality of multiplexers 1403 are converted to be different. In the configuration shown in FIG. 2B, the wavelengths of respective optical signals to be input to each of the multiplexers 1403 are converted into .lambda.1 through .lambda.n. Therefore, in the optical signal processing unit of the wavelength converting type, the input optical signal which initially has the wavelength .lambda.1 is not always output as a signal whose wavelength is .lambda.1 when it is output from each of the multiplexers 1403 to a transmission line. Which of the wavelengths .lambda.1 through .lambda.n an optical signal has may vary depending on which wavelength the wavelength converting unit 1405 converts the optical signal into.
The optical switch units shown in FIGS. 2A and 2B are normally implemented by arranging 8.times.8 optical switches in a plurality of stages.
However, the optical signal processing unit can be configured without using an optical switch.
FIGS. 3A and 3B exemplify the configurations where the optical signal processing unit is implemented by using a wavelength selecting unit.
FIG. 3A exemplifies the configuration of the optical signal processing unit of the fixed wavelength type, to which a wavelength-multiplexed optical signal is input unchanged, for routing and outputting an optical signal having multiplexed wavelengths.
The wavelength-multiplexed optical signal is directly input from an input line 1500 to a wavelength selecting unit 1501, and is routed. Inside the wavelength selecting unit 1501, the optical signals are routed depending on each of the wavelengths. However, the wavelengths are not converted. Accordingly, an optical signal having a particular wavelength is routed as it is, wavelength-multiplexed, and is output to an output line 1502.
FIG. 3B exemplifies the configuration of the optical signal processing unit of the wavelength converting type. The same constituent elements as those shown in FIG. 3A are denoted by the same reference numerals.
In the configuration shown in FIG. 3B, a wavelength-multiplexed optical signal is input from the input line 1500 to the wavelength selecting unit 1501 unchanged. The wavelength selecting unit 1501 routes the wavelength-multiplexed optical signal depending on each of the wavelengths, and outputs the signals. Since the optical signal processing unit shown in FIG. 3B is of the wavelength converting type, it is not known from which output port of the wavelength selecting unit 1501 a particular optical signal input from the input line 1500 to the wavelength selecting unit 1501 will be output. That is, in the configuration shown in FIG. 3A, an optical signal having a particular wavelength is defined to pass through a particular route and optical signals having an identical wavelength are not wavelength-multiplexed. However, in the configuration shown in FIG. 3B, an optical signal is routed regardless of its wavelength. Therefore, if optical signals are coupled by making a correspondence between a particular output port of the wavelength selecting unit 1501 and one multiplexer 1504, optical signals having identical wavelengths may sometimes be coupled. Accordingly, the wavelength converting unit 1503 converts the wavelength of an optical signal and inputs the signal to the multiplexer 1504 so that the multiplexer 1504 does not couple optical signals having identical wavelengths.
As described above, also in the configuration shown in FIG. 3B, an optical signal having a particular wavelength when being input from the input line 1500 to the wavelength selecting unit 1501 does not always have the same wavelength when being transmitted from the output line 1502 to a transmission line.
The key to the implementation of the above described system is an optical switch unit or a wavelength selecting unit with a large capacity, and expandability (an increase of processing capacity without shutting down an optical signal being used. It is desirable that the amount of hardware increases in proportion to the number of increased ports from when an initial setting is made to when a capacity is increased to its maximum) is a vital factor.
As a means for expanding a capacity of a spatial switch, a multi-stage configuration, whose typical type is a three-stage configuration as proposed by Clos, is normally adopted. As a matter of course, such a configuration is also applied to an optical switch circuit network.
The configuration of the wavelength selecting unit in the wavelength-multiplexed optical XC system, for example, the configuration of the wavelength converting type is disclosed by Japanese Laid-open Patent Application (TOKUGANHEI) No. 8-019964 in detail.
In a conventional configuration, for example, as the method for expanding a three-stage circuit, the maximum number of switches in the second stage are installed from the start of the system, and the number of pairs of switches in the first and third stages is sequentially increased. As the method for expanding a five-stage circuit, the maximum number of switches in the third stage are installed from the start of the system, and the number of pairs of switches in the first and fifth stages, and the number of pairs of switches in the second and fourth stages, are respectively and sequentially increased.
For such a multi-stage switch circuit network, the maximum number of switches in an intermediate stage must be installed from the start of the system. Therefore, the amount of hardware at an initial setting becomes large, which leads to a problem in expandability.
However, in an optical switch circuit network, the loss in an optical signal (decrease of a power level) becomes larger as the switching capacity increases. Accordingly, an optical amplifier for compensating for the loss must be arranged in the switch circuit network. Also in such a case, an optical amplifier or optical amplifiers must be arranged in consideration of expandability, and the number of optical amplifiers should be sequentially increased according to the expansion of the switching capacity.
Also in the wavelength selecting unit, the loss in an optical signal (decrease of a power level) becomes larger as the processing capacity increases. Therefore, an optical amplifier must be arranged in the wavelength selecting unit in order to compensate for the loss. Also in this case, an optical amplifier or optical amplifiers must be arranged in consideration of the expandability, and the number of optical amplifiers should be sequentially increased according to the expansion of the processing capacity.